The grant "Sequence Studies of the gp70's of Recombinant Retroviruses" is directed toward elucidating in structural terms the role of recombinant viruses in murine leukemia. The rationale for these studies is derived from findings which indicate that oncogenic viruses arise during the transition from the pre-leukemic to leukemic state in the high leukemia incidence AKR mouse strain. These viruses have been shown to be recombinant derived from otherwise innocuous endogenous retroviruses present in the mouse genome. The site of recombination is within the envelope gene encoding the surface glycoprotein in gp70. The purpose of our study is to compare the amino acid sequences of the gp70's of the AKR parental recombinant viruses to determine what changes have taken place and to try to establish structural correlates among retroviruses which induce leukemia. Our work has concentrated on deriving specific fragments unique to the recombinant virus gp70's. In certain instances, these fragments have been obtained and we are now in the process of making the above mentioned comparisons.